Subject: Variables for Controlling the Continuity Error in InfoSWMM compared to SWMM 5
Time step is always a key parameter in SWMM 5 as it is the main parameter for a user to adjust in case of a significant continuity error. InfoSWMM has additional flexibility and allows the user to control the number of Picard iterations and the Node continuity stopping tolerance. SWMM 5.0.022 has the number of iterations fixed at 8 and the stopping tolerance fixed at 0.005 feet. The stopping tolerance is important because it controls the number of iterations during a time step. For example, if all of the Nodes have a current and former iteration depth difference (absolute) less than 0.005 feet then the time step is deemed converged and the node and link flow computations are stopped for that time step. In SWMM 5 since the stopping tolerance is fixed then your main option is to reduce the time step. In InfoSWMM you can also decrease the stopping tolerance, increase the number of iterations or decrease the time step. In most models it is best to alter all three parameters for the fastest model. For example, a slightly smaller maximum time step or a smaller variable time step adjustment factor, more iterations and a smaller tolerance will normally work better than just lowering the time step.